In industrial products or materials, such as latex emulsions, water base paints, paper coating compositions, textile oils, metal processing oils and electric insulator materials, in paper and pulp industries and in cooling water systems in various industries, troublesome microorganisms, such as bacteria, fungi and yeasts, can readily grow and cause quality deterioration, for example, decay, change in quality or contamination, of products, or productivity reduction due to slime formation. To prevent the growth of, or control, such troublesome microorganisms, a variety of agents, for example, organometallic compounds, organosulfur compounds, quaternary ammonium compounds and phenolic compounds, have been used so far. However, such compounds have disadvantages; for example, they have toxicity or environmental pollution problems or cause foaming or product quality deterioration in some specific instances. When evaluated as biocides for industrial use, they are not fully satisfactory from the efficacy viewpoint; they are not so effective against certain kinds of microorganism or their efficacy cannot last for long.
To solve such problems encountered with the conventional agents, such as mentioned above, the present inventor has already proposed that a mixture of 5-chloro-2-methyl-3-isothiazolone and 2-methyl-3-isothiazolone (hereinafter referred to as "isothiazolone mixture") should be added, in the metal complex form, to water systems, together with 4,5-dichloro-1,2-dithiol-3-one, to thereby achieve effective slime control (Japanese Kokai Tokkyo Koho No. 1105/1985).
This proposal was based on the finding that the combined use of the active ingredients mentioned above can produce a remarkable synergistic effect. Regrettably, however, the isothiazolone mixture in the metal complex form, one of the active ingredients to be used combinedly, is available only in the form of a complex with an alkaline earth metal compound, such as magnesium chloride, and in the form of an aqueous solution containing magnesium (II) nitrate, which serves as a stabilizer, and, in addition, the other active ingredient, namely, 4,5-dichloro-1,2-dithiol-3-one, is fairly unstable in the presence of water, undergoing hydrolysis, which leads to complete loss of its biocidal activity. Furthermore, the additional presence of an alkali metal salt promotes the decomposition of the dithiol compound. It is thus practically impossible to have a stable slime control composition containing the active ingredients mentioned above, namely, 4,5-dichloro-1,2-dithiol-3-one and the isothiazolone mixture in the metal complex form. Accordingly, in practical application, it is necessary to add combinedly two preparations containing the respective ingredients singly to treatment targets. Such manner of handling a biocidal composition is disadvantageous or troublesome. In addition, in such combined use, the isothiazolone mixture in the metal complex form and the stabilizer magnesium nitrate can cause aggregation of colloidal pitch (resin) in white water in papermaking processes. The resulting adhering resin is a cause of troubles. In spite of its being based on the excellent synergy found with the active ingredients mentioned above, the earlier invention thus has limitations as far as the practical application method and treatment targets are concerned.